Conventionally, a PS plate having a construction such that a lipophilic photosensitive resin layer is provided on a hydrophilic support has been widely used as a lithographic printing plate. As for the plate-making method, a desired printing plate is usually obtained by performing mask exposure through a lith film and then dissolving and removing the non-image area.
In recent years, digitization techniques of electronically processing, accumulating and outputting image information using a computer have been widespread and various new image output systems corresponding thereto have been put into practical use. As a result, a computer-to-plate (CTP) technique of directly producing a printing plate without using a lith film but by scanning high directivity light such as laser light based on the digitized image information is demanded and it is now an important technical subject to obtain a printing plate precursor suitable for such a technique.
As one of the means for obtaining such a lithographic printing plate capable of scanning exposure, a construction such that a photopolymerizable composition having excellent photosensitive speed is used as an ink-acceptable photosensitive resin layer (hereinafter referred to as a “photosensitive layer”) provided on a hydrophilic support has been heretofore proposed and this is already available on the market. The printing plate precursor having such a construction is facilitated in the development processing and favored with good plate-making and printing performances of giving excellent resolution, inking property, press life and scumming resistance.
The photopolymerizable composition fundamentally comprises an ethylenically unsaturated compound, a photopolymerization initiation system and a binder resin, where the photo-initiation system absorbs light to produce an active radical and the radical induces the addition polymerization of ethylenically unsaturated compound, as a result, the photosensitive layer is insolubilized, thereby effecting the image formation. Most of conventional proposals regarding the photopolymerizable composition capable of being subjected to scanning exposure use a photo-initiation system having excellent photosensitivity and many compositions therefor are described, for example, in Bruce M. Monroe et al., Chemical Review, 93, 435 (1993) and R. S. Davidson, Journal of Photochemistry and Biology A: Chemistry, 73, 81 (1993).
For the purpose of elevating productivity in the plate-making step, it is demanded to perform the writing at a higher speed, however, conventional CTP systems using a photopolymerizable composition comprising the above-described initiation system and using a long wavelength visible light source such as Ar laser (488 nm) or FD-YAG laser (532 nm) as the light source cannot satisfy such a requirement because the output of light source or the sensitivity of photosensitive material is not sufficiently high.
On the other hand, a semiconductor laser using, for example, an InGaN system material and capable of continuous oscillation in the region from 350 to 450 nm has been recently put into practical use. The scanning exposure system using such a short wavelength light source is advantageous in that since the semiconductor laser can be produced at a low cost in view of its structure, an economical system can be constructed while having sufficiently high output. Also, as compared with conventional systems using an FD-YAG or Ar laser, a photosensitive material having a photosensitive range in the shorter wavelength region and enabling working under brighter safe light can be used.
In the imaging field, the technique for obtaining a photo-initiation system having high sensitivity is still keenly demanded as described, for example, in J. P. Faussier, Photoinitiated Polymerization-Theory and Applications: Rapra Review, Vol. 9, Report, Rapra Technology (1998) and M. Tsunooka et al., Prog. Polym. Sci., 21, 1 (1996).
However, a photo-initiation system having sensitivity sufficiently high for the scanning exposure in the short wavelength region of 350 to 450 nm is not known to date.
Conventionally, an initiation system comprising a combination of a specific dye and a titanocene compound is known as a relatively high-sensitive photo-initiation system. A combination of a dye having an oxazolidine acidic nucleus with a triazine photo-initiator is disclosed in JP-B-61-9621 (the term “JP-B” as used herein means an “examined Japanese patent publication”) but this compound is poor in the storage stability and has a problem in the production. Also, a combination of a dye having an oxazolone acidic nucleus with titanocene is disclosed in JP-A-8-272096 (the term “JP-A” as used herein means an “unexamined published Japanese patent application”) and a combination of a dye having a 5-membered heterocyclic acidic nucleus with titanocene is disclosed in JP-A-10-101719. These compounds have certainly high sensitivity but the sensitivity is still not satisfied and when a laser light source having a wavelength of 450 nm or less is used, a sufficiently high sensitivity cannot be obtained in practice. Thus, these compounds are not suitable for use with a short wavelength light source.